Living things respire. Respiration is a complex sequence of chemical reactions
which result in the release of energy from food. There are two types of respiratory
process:

Aerobic respiration

Carried out by the vast majority of organisms, this involves
oxygen. The by-products of the reaction are water
and carbon dioxide both of which are eliminated
as waste products. Oxygen is obtained from the air or water
using organs designed to optimise gaseous exchange.
These include the stomata in plants (small, size regulated
pores), spiracles in arthropods, gills in fish and lungs
in mammals. The uptake of oxygen and simultaneous elimination
of carbon dioxide and water is commonly referred
to as breathing. It is important to distinguish between
breathing and respiration. It is tempting, particularly
with younger children to use the well used term breathing
as an all embracing description of the respiratory process.
However, this is not correct and could lead to the reinforcement
of misconceptions.

Anaerobic respiration

When oxygen levels are at a low level, it is possible for
some simpler organisms and parts of more complex ones to
release energy from food without oxygen. This is a far less
efficient process but a necessary alternative in some cases.
The by-products of anaerobic respiration are different to
aerobic. In humans, oxygen starved muscle cells will
respire anaerobically under stress such as heavy physical
activity. The by-product of this is lactic acid and it is
this that causes the puffed out feeling. Yeast cells
respire anaerobically in sugar solution producing alcohol
as the by-product.